Gastrula

GASTRULA (from the Greek gaster - stomach), an embryological term introduced by Haeckel to designate the 3rd stage of development following the blastula stage (see); the process of formation of the gastrula is called gastrulation. The embryo in the gastrula stage has two germ layers: the outer (ectoderm, ectoblast) and the inner (endoderm, entoblast); in typical cases, the gastrula is a two-layered pouch enclosing a cavity, the so-called archenteron (primitive gut), communicating with the external environment through the blastopore (see). Such is the gastrula of amphioxus and certain invertebrates, for example, sea urchins (see Figure 1 in the article Blastopore). It is formed by invagination, i.e., the indrawing of the blastula wall, which turns inward until it touches the opposite wall; the lower region of the blastula, formed by larger blastomeres, invaginates; the mechanism of invagination reduces to cell proliferation along the edge of this region. The gastrula of amphibians is constructed on the same plan (see Figure 2 in the article Blastopore), only at the bottom of the cavity lies a large number of large, yolk-rich blastomeres, which, emerging through the blastopore, form Rusconi's yolk plug; this gastrula is formed by a complex process of invagination and simultaneously epiboly, i.e., the overgrowing from the outside of the vegetative pole of the blastula by the small cells of the animal pole. The gastrula of reptiles and birds consists of a two-layered germ disc (blastoderm, see) covering the upper pole of the unsegmented yolk like a watch glass; between the disc and the yolk lies a cleft (subgerminal cavity) corresponding to the primitive gut, but not communicating with the external environment (see figure in the article Blastoderm). Attempts to explain the origin of this gastrula by invagination have now been abandoned, and the depression (blastopore) observed at the posterior end of the blastoderm in reptiles is associated with the formation of the mesoderm and notochord, i.e., with the process of chordulation; the formation of the two-layered embryo, i.e., the process of gastrulation as such, proceeds by delamination (i.e., the splitting of the cell layer into two sheets) or by immigration, the ingrowth of scattered vegetative cells beneath the outer layer and their subsequent fusion into the inner layer. In mammals, corresponding to the blastula stage, a trophoblast vesicle is formed, to the inner wall of which a cell cluster—the embryonic knot—adheres, giving rise through splitting to the outer and inner layers. The details of the process in different species are not identical: in humans, for example, the embryonic knot first forms two vesicles—the ectodermal (amnion) and entodermal (yolk)—which at the point of contact form a two-layered embryo. According to Haeckel's biogenetic law, the gastrula stage in ontogeny corresponds in phylogeny to the hypothetical free-living gastrea form, common to all multicellular organisms and similar to the gastrula of amphioxus.